The invention is directed to a high efficiency ultra-violet (UV) responsive negative electron affinity photocathode with the long wavelength cutoff tunable over the wavelength from .about.200 to .about.360 nm based on Al.sub.x Ga.sub.1-x N.
The III-V semiconductor alloy system Al.sub.x Ga.sub.1-x N has several important potential advantages as a UV photocathode material:
The long wavelength cutoff can be varied from .about.200 nm to .about.360 nm. PA1 It has a very large absorption coefficient PA1 The photoelectron emission quantum efficiency is higher than homogenous solids because of the ability to tailor the electronic band structure near the surface with the use of heterostructures. PA1 Negative electron affinity photocathodes, for sharply enhanced photoemission yield, can be formed by applying a layer of cesium to the surface of Al.sub.x Ga.sub.1-x N for which the Fermi energy level is appropriately positioned. PA1 It can be configured as a transmission photocathode or a front side illuminated photocathode.
Al.sub.x Ga.sub.1-x N is a direct bandgap semiconductor which can be grown in single crystal form on sapphire substrate. It will not be sensitive to visible radiation since it has a well defined long wavelength absorption edge characteristic of a direct bandgap semiconductor. The measured optical absorbance shows an increase of 4 orders of magnitude over a wavelength range of approximately 20 nm at the absorption edge.
Al.sub.x Ga.sub.1-x N is an alloy of AlN and GaN. The composition of the alloy can easily be varied during growth. By varying the composition, x, the bandgap and hence the long wavelength absorption edge can be varied from .about.200 nm to .about.360 nm. Other commonly used photocathode materials such as CsTe have a fixed absorption edge which may not be a good match for some applications. The control of aluminum composition is achieved simply by the mass flow control of hydrogen through the Ga and Al metal organic sources during growth.
Thus the ability to tailor the band shapes at or near the surface provides an attractive degree of freedom in enhancing photoelectron escape probability.
Al.sub.x Ga.sub.1-x N has a very large absorption coefficient characteristic of direct bandgap semiconductors such as GaAs. In fact the absorption coefficient in Al.sub.x Ga.sub.1-x N is expected to rise even more sharply near the edge than in GaAs since the electron effective mass and hence the density of states is larger. In contrast, the amorphous photocathode materials typically have a relatively soft absorption edge.